Extruder comprising blister mechanism

ABSTRACT

The invention relates to an extruder in particular for processing polymer melts, with an extrusion space ( 4 ) which is arranged in an extruder barrel and through which at least one extruder screw shank ( 2 ) extends in such a way that it can be rotated by a motor, said screw shank having on it at least one section ( 5 ) which is in the form of a cylinder casing, the extrusion space ( 4 ) having at least one barrel section ( 1 ) which surrounds in an essentially equidistant manner the at least one extruder screw shank ( 2 ) in the area of the at least one region ( 5 ) in the form of a cylinder casing to form an encircling gap ( 17 ) as a restricted flow zone (blister  15 ), in particular to set the pressure of the melt. In this arrangement, the clearance of the encircling gap ( 17 ) at the restricted flow zone is able to be changed in a specifically selective manner by externally adjustable force, with elastic deformation of the inside surface of the extrusion space ( 4 ) and/or of the surface of the screw shank ( 2 ), by a hydraulic pressure chamber ( 18 ) being provided in the region of the restricted flow zone in the wall of the barrel section ( 1 ) and/or in the least one screw shank ( 2 ), which pressure chamber has on the side facing the extrusion space ( 4 ) a considerably smaller wall thickness (membrane wall  19 ) in comparison with the wall thickness of the extrusion space ( 4 ), and by a hydraulic pressure unit being provided, supplying the force for the surface deformation.

DESCRIPTION

[0001] The invention relates to an extruder, in particular forprocessing polymer melts, with an extrusion space which is arranged inan extruder barrel and through which at least one extruder screw shankextends in such a way that it can be rotated by a motor, said screwshank having on it at least one section which is in the form of acylinder casing, the extrusion space having at least one barrel sectionwhich surrounds in an essentially equidistant manner the at least oneextruder screw shank in the area of the at least one region in the formof a cylinder casing to form an annular gap as a restricted flow zone(blister), in particular to set the pressure of the melt.

[0002] An extruder of this type is disclosed in U.S. Pat. No. 4,332,481.

[0003] Such a restricted flow zone formed as an encircling gap, forexample as an annular gap, in the extrusion space is also referred to asa blister and serves in particular for setting the pressure of the meltprocessed in the extruder. Downstream of this restricted flow zone,entraining agents may be added for example, being incorporated into theexpanding polymer melt and serving as stripping agents. The effect ofthe blister is strongly dependent on the clearance in the annular gapbetween the cylindrical part of the extruder screw and the insidesurface of the cylindrical extrusion space. In practice, the clearanceis often of the order of approximately 3 mm. The optimum clearance isdependent less on the size of the respective extruder than on theproperties of the material to be processed (temperature, viscosity).According to the prior art, changing of the clearance often involves acorresponding change of the screw shank or the cylindrical part of thescrew shank in the region of the restricted flow zone. The processing ofdifferent materials therefore frequently requires the screw shank to bechanged in order that best possible processing conditions can be set. Inthe case of non-steady-state processes during processing, the optimumsize of the annular gap of the blister changes. Until now there has beenno possible way of optimizing the clearance in such a situation. Inparticular during start-up operations, in which the extruder has not yetreached a constant operating temperature, changes in the effectiveclearance of the blister may be brought about by different degrees ofthermal expansion. It has so far also not been possible to deal withthis in an optimum way.

[0004] DE 198 31 540 A1 discloses a device for partially changing aclosed flow channel cross section, the inner contour of which is formedby a displacement element, for example of a cylindrical shape. The flowchannel is preferably part of an extrusion die for producing a tube fromflowable compositions. The device has external adjusting elements in theform of pins which can be adjusted radially from the outside inward andat least one region of a flow channel wall which comprises a sleeve witha wall thickness of at most 1 mm, the sleeve being locally deformableinward by means of the adjustable pins. The displacement elementarranged in the flow channel may likewise be provided with a sleeve of asmall wall thickness, which is locally deformable outward by means ofsupporting elements that are axially displaceable on conical supportingsurfaces. In this way, the flow channel cross section can be reduced ina specifically selective manner in the region of the thin flow channelwall. A disadvantage of this known device is that the adjustable pinsand displaceable supporting elements do not allow uniform deformationover the cross section of the flow channel, since the introduction ofthe deforming forces can only ever take place in a punctiform or locallylimited way. The greater the number of elements used for thedeformation, the more uniformly the deformation can be set, but thegreater the actuating effort for these elements as well. Anotherdisadvantage is that the thin walls of the sleeves are unavoidablydeformed by the melt pressure prevailing in the flow channel counter tothe force of the elements used for the adjustment. Consequently, it isimpossible for the device to be operationally used under geometricalconditions of the entirely unloaded state (zero position).

[0005] Further possibilities emerge from JP-60002329, where regions ofan extruder barrel which can be elastically deformed by adjustingdevices and interact with screw segments to produce an adjustablerestricting gap are provided, or from U.S. Pat. No. 3,122,789, whichshows a membrane which can be elastically deformed by a pressure fluidto adjust the flow channel height in a slot die.

[0006] It is an object of the invention to develop an extruder of thetype stated at the beginning to the extent that a largely optimum modeof operation in the region of the blister is possible even if differentmaterials are to be processed or the material properties change, withoutany major modification work being required for the extruder. The effortfor the adjustment of the blister is to be as little as possible.

[0007] This object is achieved in the case of an extruder of the generictype by the clearance of the encircling gap at the blister being able tobe changed in a specifically selective manner by externally adjustableforce, with elastic deformation of the surface of the extrusion spaceand/or of the screw shank. For this purpose, the invention provides ahydraulic pressure unit which supplies the force for the surfacedeformation in the region of the restricted flow zone either on theinside surface of the extrusion space or on the cylindrical surface ofthe at least one screw shank or else on both, a hydraulic pressurechamber being provided in the region of the restricted flow zone in thewall of the barrel section where the blister is to be arranged and/or inthe cylindrical part of the at least one screw shank. The pressurechamber has on the side respectively facing the extrusion space aconsiderably smaller wall thickness in comparison with the wallthickness of the extrusion space. This wall is referred to hereafter asa membrane wall because of its small thickness.

[0008] The pressure chamber is expediently provided respectively in saidbarrel section of the extruder barrel and not on the screw shank. Thereason for this is primarily to be seen in the fact that, because of theassociated sealing problems, it is more difficult to provide a supply ofhydraulic pressure medium to the pressure chamber on the screw shankthan on the extruder barrel, which unlike the screw shank is completelyimmovable.

[0009] The invention can be realized particularly advantageously in thecase of a single-screw extruder, which has a purely cylindricalextrusion space, with the result that the encircling gap at the blisteris formed as an annular gap. The hydraulic pressure in the pressurechamber allows the membrane wall to deform to the respectively desireddimensions in a geometrically very uniform shape (circular shape). Thedesired settings can be controlled extremely quickly and easily, sinceall that matters is the difference in pressure between the hydraulicpressure and the pressure in the extrusion space.

[0010] However, the invention can also be applied very advantageously inthe case of twin-screw extruders. In these extruders, the extrusionspace is formed by two longitudinal bores which overlap partially incross section, at least over part of the axial length of the extruderbarrel, and because of their shape are also referred to as a“spectacle-bore”. Arranged in this spectacle-bore are two co-rotatableor counter-rotatable motor-driven screw shanks which lie parallel toeach other and each have at least one cylindrical section. Thecylindrical sections of the two extruder screw shanks in this caserespectively lie directly next to each other. If the pressure chamber isnot arranged respectively on the two extruder screw shanks but isprovided on the inside surface of the barrel of the extrusion space,that is to say in the spectacle-bore, it is recommendable not to formthe pressure chamber as a completely encircling chamber but to divide itinto two separate chambers which are arranged mirror-symmetrically inrelation to each other and, in the cross section of the extruder barrel,extend only in each case over by far the greater part of the cylindricalsurfaces of the longitudinal bores (spectacle-bore), with the twointerstitial regions being cut away. Consequently, in the transitionalregion between the two longitudinal bores, the two chambers are notcontiguous.

[0011] A particularly advantageous embodiment of the invention in thecase of a twin-screw extruder is obtained by providing a special barrelsection which, instead of a spectacle-bore, has two corresponding fullycylindrical, i.e. not overlapping, longitudinal bores for the two screwshanks, these longitudinal bores of course having a smaller diameterthan the respective partially cylindrical overlapping bores of thespectacle-bore. The diameter of the fully cylindrical bore preferablycorresponds in each case to the root diameter of the respective screwshank, that is to say is only slightly larger than the latter. In thisembodiment, in this special barrel section there is consequently amaterial bridge between the two longitudinal bores at the locationcorresponding to the interstitial region of the spectacle-bore.Respectively arranged in the surface of the longitudinal bore in thisbarrel section is a hydraulic pressure chamber, which is designed suchthat it runs all the way around in the same way as in the case of theembodiment for a single-screw extruder. This solution permits acompletely uniform setting of the respective annular gap over the entirecircumference. The pressure chambers of the two longitudinal bores canbe partitioned off fully from each other, but may also be directlyconnected to each other in conducting terms.

[0012] Since the parts of the screw shanks provided with flights have agreater outside diameter than the fully cylindrical longitudinal boresof the special barrel section, they cannot be pushed through the latterduring assembly. Therefore, either the barrel section must be of adivided design, which would correspondingly also require division of thehydraulic pressure chamber in the longitudinal bore, or the screw shankis respectively divided into axial portions which can be coupledtogether, for example by means of plug-in connections. It is alsopossible to use screw elements with a central through-bore, which arepushed into the extruder barrel from both ends of the latter and aresubsequently securely connected to each other by means of a tension rodpassed through the through-bore.

[0013] It is recommendable to arrange the pressure chamber of theblister in a separate, essentially annular part of the barrel which hasonly a short axial length, consequently the length of which is less thanthe inside diameter of the extrusion space. This annular barrel part isexpediently connected to the other parts of the extruder barrel by aflange connection.

[0014] It is conducive to aspects of production engineering if thepressure chamber is made as a welded structure. In this case, the weldsshould advantageously be provided outside the membrane wall inthicker-walled regions, in order as far as possible not to allow themembrane wall itself to get into the zone of thermal influence of theweld, which could change its material properties unfavorably.

[0015] The membrane wall should be dimensionally designed for adifference in pressure of at least 100 bar, preferably of at least 150bar. In any case, it must be ensured that the forces on the membraneinduced by the hydraulic pressure applied in the pressure chamber leadonly to elastic deformations, but not to plastic deformations.Furthermore, the loading should lie below the critical buckling load, inorder that there is no wave-like deformation in the circumferentialdirection on the membrane wall which is arranged on the inside surfaceof the extrusion space. This is because it would mean that the effectiveclearance of the blister varies in the circumferential direction. Inpractice, the expedient thickness of the membrane wall lies in a rangefrom approximately 0.5 to 3 mm, preferably in the range from 1 to 2 mm.However, these figures are not to be regarded as restrictive. Inprinciple, the degree of deformation not only depends on the thicknessof the membrane wall and the applied pressure in the pressure chamberbut is of course also strongly influenced by the axial length of themembrane wall, i.e. by the length of the extent of the pressure chamberin the direction of the axis of the screw shank. The longer the membranewall, the greater the deformation under the same pressure.

[0016] The invention can be used with particular advantage on anextruder in which a multi-screw extruder section is arranged downstreamof the blister in the direction of material flow. This multi-screwextruder section may be designed in particular as a planetary-gearextruder section.

[0017] By providing an extruder with, a blister in the way according tothe invention, allowing the effective clearance of the annular gap to bechanged unproblematically at any time in a simple, specificallyselective manner by elastic deformation through a force which can beadjusted from outside, it is possible without changing the screw shankto process different materials under optimum process conditions,provided that the remaining screw shank elements are suitable inprinciple for the respective material. In the case of non-steady-stateprocessing conditions, it is possible in the course of operation to makean optimum choice of the clearance of a blister and realize it. In thecase of a blister according to the invention, with a hydraulic pressurechamber and membrane wall, this is possible by simply presetting thehydraulic pressure in the pressure chamber or the difference in pressurewith respect to the pressure in the extrusion space.

[0018] The invention is explained in more detail below on the basis ofexemplary embodiments in the drawing, in which:

[0019]FIG. 1 shows a schematic representation of an extruder with aplanetary-gear extruder section arranged in the middle part,

[0020]FIG. 2 shows a schematic representation of a detail of a blisterwith a hydraulic pressure chamber,

[0021]FIG. 3 shows a part of the barrel with a hydraulic pressurechamber,

[0022]FIG. 4 shows a cross section through the extruder barrel of atwin-screw extruder according to the invention and

[0023]FIG. 5 shows a modification of the embodiment of FIG. 4.

[0024] The schematic representation of the extruder in FIG. 1 shows anextruder barrel which is passed through by a screw shank 2 overvirtually the entire axial length. The screw shank 2, provided with ajournal 3 with fitting key, is rotationally driven by a motor (notrepresented) via a gear mechanism (likewise not represented). The barrelsection 1 which directly adjoins the gear mechanism and the cylindricalsurface of which essentially corresponds in diameter to the outsidediameter of the screw of the screw shank 2 encloses a part of theextrusion space 4. The barrel section 1 is provided with a connectingpiece of a melt feed 14. Consequently, the extruder represented is fedthe material to be processed not in the form of granules or powder butin an already molten form. Directly following the screw of the screwshank 2 there is also arranged in the barrel section 1 a blister 15,which forms a restricted flow zone, so that the part of the extrusionspace 4 lying upstream of the blister 15 is always completely filledwith melt. The blister 15 is formed by a short section 5 on the screwshank 2 in the form of a cylinder casing. On both sides of the section 5in the form of a cylinder casing there likewise adjoin shortfrustoconical regions of the screw shank 2. This design of the blister15 is discussed in still more detail further below.

[0025] Arranged downstream of the blister 15 in the barrel section 1 isthe connecting piece of an entraining-agent feed 13, through whichwater, for example, can be jetted into the extrusion space 4 andconsequently into the melt. The screw shank 2 is designed downstream ofthe blister 15 as a so-called porcupine 6, i.e. is provided withspike-like mixing elements protruding radially from the screw shank 2.In this region of the extrusion space 4 there is therefore considerableformation of bubbles in the melt. The melt then passes into aplanetary-gear extruder section 8, two of the planetary spindles 9 ofwhich can be seen in the representation. The planetary spindles 9 arerespectively mounted at their ends in the planetary gears 7 a, 7 b of aplanetary gear mechanism, the sun gears of which are denoted by 10 a and10 b. The sun gears 10 a, 10 b constitute part of the screw shank 2. Astripping of the melt takes place in the region of the planetary-gearextruder section 8. The gases released can escape through a vent 16.Following the planetary-gear extruder section 8, there is in turn a zone11 of a single-screw extruder in which the degassed melt is brought tothe required extrusion pressure and is forced out through an extrusiondie 12.

[0026]FIG. 2 shows a detail of the region of the blister 15 designedaccording to the invention, as a section along the extruder axis. Of thescrew shank 2, only the section 5 in the form of a cylinder casing, withthe two frustoconical regions directly adjoining it, is represented. Theannular barrel part 20 of the blister 15 with the hydraulic pressurechamber 18 is designed as a short separate part of the barrel. Thepressure chamber is formed by a welded structure which is composed oftwo parts. The outer annular part is made as a solid component, whilethe hydraulic pressure chamber 18 has been machined into the innerannular part, for example by a turning operation. By inserting the twoannular parts one into the other and welding right around the annularjoining gap on both sides, the pressure chamber 18 is completely sealedfrom the outside. The hydraulic pressure medium can be introduced intothe pressure chamber 18 through a supply line that is not represented inthe sectional diagram. Toward the extrusion space 4, the pressurechamber 18 has a wall, which is referred to as membrane wall 19 and hasan extremely small wall thickness in comparison with the wall thicknessof the extruder barrel. Formed between the surface of the section 5 inthe form of a cylinder casing of the screw shank 2 and this membranewall 19 is a relatively small annular gap, which has for example a sizeof 2 or 3 mm. The size of this annular gap 17 is of great importance forthe setting of the melt pressure. Even relatively small changes in theclearance have a very noticeable effect. In order that the screw shank 2lies exactly coaxially in this annular gap 17, and in the extrusionspace 4, it is generally necessary to mount the screw shank 2 not in acantilevered manner but at both ends. The introduction of a hydraulicmedium under increased pressure into the pressure chamber 18 allows themembrane wall 19 to be deformed in the direction of the screw shank 2.The membrane wall 19 bows out downward in the axial longitudinal sectionrepresented. Seen in cross section with respect to the longitudinalaxis, this means that the inside diameter of the extrusion space 4 isreduced in the region of the membrane wall, and so the annular gap 17 ofthe blister is likewise reduced. Since the pressure of the pressuremedium can be changed within broad limits, but every pressure change isonly accompanied by comparatively small changes in the deformation ofthe membrane wall 19, the effective clearance in the annular gap 17 canbe set extremely accurately. The control effort required for this can bedescribed as small.

[0027]FIG. 3 shows by way of example, partly in side view and partlylikewise as an axial longitudinal section (subfigure a) and also as apartially broken-open perspective view (subfigure b), the barrel part 20for a blister according to the invention in a realistic form with regardto the relative size. The barrel part 20 is designed essentially in anannular or disk-shaped form. The cylindrical opening lying around theregion of the center axis is part of the extrusion space 4. In the outerregion, the barrel part 20 is provided in the manner of a flange withthrough-bores (not represented), to permit fastening to the adjoiningparts of the extruder barrel in a simple way. As distinct from theembodiment in FIG. 2, the annular pressure chamber 18 with the membranewall 19 is not shaped mirror-symmetrically in the axial longitudinalsection, but instead has a cross-sectional form resembling a shoe. Thismakes it possible to extend the pressure chamber 18 into the region ofthe offset pointing to the right of the otherwise disk-shaped barrelpart 20. In this case as well, the pressure chamber 18 is formed as awelded structure. The welds are arranged in such a way that they lie inthicker-walled regions of the barrel part 20 and not in the directvicinity of the membrane wall 19. To supply the pressure chamber 18 withhydraulic medium, a thread for a hydraulic connection 23 is provided onthe outside of the barrel part 20 and is for its part connected to thepressure chamber 18 via a channel 22. In the present case, the insidediameter of the extrusion space 4 is approximately 90 mm, while thethickness of the membrane wall 19 is around 1 mm and the length of thepressure chamber or of the associated barrel part 20 is aroundapproximately 30 mm.

[0028] Represented in FIG. 4 is a schematic section through the barrelof a twin-screw extruder, the extrusion space 4 of which is formed bythe partially overlapping longitudinal bores 25 a, b. The two extruderscrew shanks are not represented. The hydraulic pressure chamber 19 isnot formed all the way round the inside surface of the extrusion space 4but instead is composed of the two chambers 24 a, b, which arepartitioned off from each other and respectively extend over by far thegreater part of the circumference of the longitudinal bore 25 a and 25b, respectively. Only the two interstitial regions 26 a, b of thespectacle-bore are not taken up by the hydraulic pressure chamber 18.Consequently, when the hydraulic pressure chamber 18 is activated, onlya relatively small part of the surface of the extrusion space in theregion of the blister is excluded from the attempted deformation.However, this has virtually no adverse effects on the process control inthe twin-screw extruder.

[0029] A particularly advantageous modification of the embodiment ofFIG. 4 is represented in FIG. 5. In this case, instead of the twooverlapping longitudinal bores 25 a, b, the twin-screw extruder isprovided over part of its axial length, in a special barrel section 27,with fully cylindrical, that is to say not overlapping, longitudinalbores 28 a, b, arranged in an appropriately coaxial manner in relationto the spectacle-bore. In a way similar to that represented in FIGS. 2and 3 for a single-screw extruder, a completely encircling pressurechamber 29 a, b is respectively arranged in these longitudinal bores 28a, b. As in FIG. 4, the hydraulic supply line is not represented. Listof designations:  1 barrel section  2 screw shank  3 journal  4extrusion space  5 section in the form of a cylinder casing  6 porcupine 7a, b planetary gears  8 planetary-gear extruder section  9 planetaryspindle 10a, b sun gear 11 single-screw extruder zone 12 extrusion die13 entraining-agent feed 14 melt feed 15 blister 16 vent 17 encirclinggap (annular gap) 18 hydraulic pressure chamber 19 membrane wall 20annular barrel part 22 channel 23 hydraulic connection 24a, b chamber25a, b longitudinal bore 26a, b interstitial region 27 barrel section28a, b fully cylindrical longitudinal bore 29a, b hydraulic pressurechamber

1. An extruder, in particular for processing polymer melts, with anextrusion space (4) which is arranged in an extruder barrel and throughwhich at least one extruder screw shank (2) extends in such a way thatit can be rotated by a motor, said screw shank having on it at least onesection (5) which is in the form of a cylinder casing, the extrusionspace (4) having at least one barrel section (1) which surrounds in anessentially equidistant manner the at least one extruder screw shank (2)in the area of the at least one region (5) in the form of a cylindercasing to form an encircling gap (17) as a restricted flow zone (blister15), in particular to set the pressure of the melt, wherein theclearance of the encircling gap (17) at the restricted flow zone is ableto be changed in a specifically selective manner by externallyadjustable force, with elastic deformation of the inside surface of theextrusion space (4) and/or of the surface of the screw shank (2), by ahydraulic pressure chamber (18) being provided in the region of therestricted flow zone in the wall of the barrel section (1) and/or in theat least one screw shank (2), which pressure chamber has on the sidefacing the extrusion space (4) a considerably smaller wall thickness(membrane wall 19) in comparison with the wall thickness of theextrusion space (4), and by a hydraulic pressure unit being provided,supplying the force for the surface deformation.
 2. The extruder asclaimed in claim 1, wherein the extruder is a single-screw extruder witha cylindrical extrusion space (4) and an encircling gap (17) formed asan annular gap.
 3. The extruder as claimed in claim 1, wherein theextruder is a twin-screw extruder with two driven extruder screw shankswhich are arranged parallel to each other and are co-rotatable orcounter-rotatable in the extruder barrel, which has as the extrusionspace (4) two cylindrical longitudinal bores (25 a, b) partiallyoverlapping in cross section, at least over part of the axial length ofthe extruder barrel.
 4. The extruder as claimed in one of claims 1 to 3,characterized in that the pressure chamber (18) is respectively providedin the barrel section (1).
 5. The extruder as claimed in claims 3 and 4,wherein the pressure chamber (18) is divided into two separate chambers(24 a, b) which are arranged mirror-symmetrically in relation to eachother and, in the cross section of the extruder barrel, extend only overthe greater part of the cylindrical surfaces of the two longitudinalbores (25 a, b), with the two interstitial regions (26 a, b) being cutaway.
 6. The extruder as claimed in claim 3, wherein the extruder barrelhas a barrel section (27) with two fully cylindrical longitudinal bores(28 a, b) for the sections in the form of cylinder casings of the twoscrew shanks, and the longitudinal bores (28 a, b) are respectivelyprovided with a completely encircling hydraulic pressure chamber (29 a,b).
 7. The extruder as claimed in claim 6, wherein the diameter of thelongitudinal bores (28 a, b) is only slightly greater than the rootdiameter of the screw shank.
 8. The extruder as claimed in one of claims1 to 7, wherein the pressure chamber (18) is arranged in a separateannular barrel part (20) of short axial length, which is preferablyconnected by a flange connection to the remaining parts of the extruderbarrel.
 9. The extruder as claimed in one of claims 1 to 8, wherein thepressure chamber (18, 28 a, b) is formed as a welded structure.
 10. Theextruder as claimed in claim 9, wherein the welds are arranged outsidethe membrane wall (19) in thicker-walled regions.
 11. The extruder asclaimed in one of claims 1 to 10, wherein the membrane wall (19) isdesigned for a difference in pressure of at least 100 bar, in particularof at least 150 bar.
 12. The extruder as claimed in one of claims 1 to11, wherein the membrane wall (19) has a thickness in the range from 0.5to 3 mm, in particular in the range from 1 to 2 mm.
 13. The extruder asclaimed in one of claims 1 to 12, wherein at least one multi-screwextruder section, in particular a planetary-gear extruder section (8),is arranged downstream of the blister (15).